My AR-2A to AR-2AX Conversion

[Guest jlaplante]

I am about to embark on my own AR-2A refurbishing project, and want to document my steps for the forum. I also have a few questions at the end of this post.

My journey started in the early 60's when my dad bought his first real hi-fi: two AR-2A speakers, an AR turntable, and a Fisher 35 w/ch tube receiver. Years later I bought my own ARs, a pair of AR-4X, which are still in the family, but badly in need of refurbishing (another day). In the early 70's we moved to another house, and dad swapped the 2A's for my 4X's since he no longer had room on bookshelves. What good fortune! I have owned those speakers since, and put many miles on them:

• Model: AR-2A
  
• Serial numbers #43275, #43284
  
• Purchased circa 1962-63?
  
• Oiled walnut, great condition
  

Around 1995, I became fed up with the crappy pots and worn out mid+tweets. I found AB Tech services, and upgraded both units to 2AX mids+tweets, replaced one woofer, grill cloth, and emblems. I soldered the pots with a jumper to simply bypass them, running the mid+tweet at full level. I did not modify anything else.

Today, I am looking to finish the job started in 1995. I just bought a nice NAD T-775 receiver with 100 w/ch. After shopping briefly for new speakers and spending time on this forum, I decided that upgrading the AR-2A's will give me the best bang for the buck. They are gorgeous with the new grill cloth and truly have a sound I can listen to for days.

STEP #1: WOOFERS

I looked briefly into upgrading the woofers to modern units, either AB tech units or better OTS units. See thread Modern Woofer for AR-2AX?.

The bulk of the opinions here indicate that the woofers in the 2A are prized, and I should just recone them to avoid changing anything drastic about the acoustics. Very few modern woofers seem to be designed like these. These are the older cast aluminum frames with the wide aluminum 6-hole flange, cloth surround, and I believe alnico magnets (?). It seems the woofer quality in later AR units also suffered from a gradual capitulation to production and warrantee cost reduction resulting in the switch to stamped steel, ceramic magnets, and increased suspension stiffness. So I feel fortunate to have at least one original woofer. By looking at my 1995 AB tech replacement unit, I can see immediately that the older woofer is just plain better built .

Yesterday I lucked out. As soon as the decision to recone was made, I found a damaged but original AR-2A woofer on eBay. Last night I became the proud owner for $4.99 + $12 shipping. I will now send both woofers to MillerSound for reconing; here is a photo:

STEP #2: MID+TWEETERS

I would like to devote the time to putting in a top-grade tweeter + mid. However I don't have the time, and the difficult crossover design process would probably defeat my whole purpose. So today I am resigning myself to keeping the AB mid+tweets for now.

If someone comes along, as Carlspeak did with his 3A Super-Mod, and spells out a well designed and tested tweeter + mid + crossover upgrade for the 2AX, then I would probably just spring for that. Is there anything like that out there?

STEP #3: CROSSOVERS

I caught wind on here somewhere that the crossover changed slightly going from the 2A to 2AX. I also see suggestions to slightly correct the tweeter in AR-3A upgrades with a small inductor in parallel, and wonder if the AB tech tweeter for the 2AX needs something similar.

I started by posting here: AR-2A to 2AX tweeter conversion - xover change needed?. In response to replies, here is my current crossover. The first mystery was the inductor value. The numbers marked on the inductor are not very clear, but appear to be "2":

To be sure, I weighed one coil to be 9.1 oz. Referring to the inductor table, it looks to actually be a #5, with a value of 1.187 uH. So assuming this is the case, here is a thorough schematic of my original AR-2A crossover:

(values of the pots are TBD, but should be well known in these parts - 16 ohm?)

Here are some photos of the actual crossover for reference. Note that the #2 lug on the mid potentiometer has no wire connected, and no sign of ever being soldered - I will get to that. Note also the June 1962 date stamped on the capacitor; this is my best clue so far regarding the purchase date:

Next, I compared my crossover to the 2AX crossover drawing posted on this site. Here is a reproduction of that drawing again, freshened up a bit in photoshop for visibility:

Right away there are differences. So I mapped out the full 2AX schematic as best I could from the drawing. Some information is missing, such as the actual polarity on the speakers and which pot terminals are which, so I made assumptions:

You will notice the areas of question circled in red.

• There appears to be an inductor change between 2A and 2AX based on info in the inductor table.
  
• The 2AX drawing clearly shows the #2 terminal of the mid-level pot connected to the T input. This is NOT the case in my 2A xover.
  
• The wire color designations on the mid & tweeter also suggest a polartiy inversion from mine. However, this seems like it must be an error, based on conventional crossover design.
  

CORRECTION:

After measuring the drivers, it appears there are impedance questions. Both AB tweeters measure in at just under 4 ohms DC, suggesting they are less than 8 ohm drivers. Likewise the old woofer is about 5.5 ohms, compared to 6.5 ohms for the AB replacement.

This is where my journey ends today.

Questions:

• Can anyone confirm or correct the 2AX schematic?
  
• Can anyone shed light on any other mods that may be required to get the AB tech mid+tweet to behave as close to stock as possible?
  
• Did the 2AX use different impedance drivers from the 2A?
  
• Has anyone designed a nice AR-2AX tweeter+mid upgrade path w/ modern components (other than AB tech)?
  

Thanks

[Guest jlaplante]

I have to add some corrections based on information from johnieo's post and schematic in thread OK, who has drawings of the AR2ax?

• It is starting to look like my schematic and photos may have the polarity of the mid and tweeter reversed. My schematic was derived from a speaker I had previously modified myself years ago with new mid+tweeter, and I possibly wired them backwards, assuming yellow as positive. The original mid+tweeters had no +- markings, is that true?
  
• I believe the tweeter impedance shown in my 2A crossover schematic should be 4 ohms. I don't have the original tweeters anymore to be sure.
  
• I mistakenly call out uH for the inductor - it should be mH
  

Does the schematic HERE suggest that black goes to positive on the mid & tweet? It is not called out. Black is certainly the positive input w/ respect to the woofer wiring in the drawing.

Does anyone know if these 2A/2AX speakers were wired with the mid+tweeter reversed in polarity?

[Guest jlaplante]

Well the more I poke around on the forum, the more answers I find to my own questions! Sorry CSP members for redundant posts!

I believe that black is wired to positive, according to a Jul 11, 2007 post by charger3834: "The leads are wired conventionally i.e. yellow is negative..." from AR 2ax crossover mystery?

[RoyC]

Does anyone know if these 2A/2AX speakers were wired with the mid+tweeter reversed in polarity?

The 2ax experienced a number of variations along the way, but all 2ax drivers I have seen were wired in same polarity. The yellow crossover wires from the (center) "B" tab of the pots go to the the negative sides of the tweeter and midrange. In the case of the later 3/4" tweeter, the lead wires are crossed over under the tape on the cabinet surface to maintain "like" polarity. The (earlier) orange dome version leads do not cross over. (The orange dome dcr, by the way, is 2 ohms.) The woofer inductor was changed from a #5 to a #7 around 1970.

As John mentioned in your other thread, your project depends on what version of 2ax you are trying to reproduce. Unfortunately using the old crossover circuit with AB Tech replacement drivers for both tweeter and mid will NOT result in anything remotely close to any version of the 2ax. The AB Tech drivers are much different than the original drivers, and essentially require a different crossover. Consequently, knowing the original polarity will not be of much assistance in this case anyway.

Roy

[Guest jlaplante]

As John mentioned in your other thread, your project depends on what version of 2ax you are trying to reproduce. Unfortunately using the old crossover circuit with AB Tech replacement drivers for both tweeter and mid will NOT result in anything remotely close to any version of the 2ax. The AB Tech drivers are much different than the original drivers, and essentially require a different crossover. Consequently, knowing the original polarity will not be of much assistance in this case anyway.

<br />

Ouch!

Thanks Roy - good advice, I was afraid of that. Thus all my fussing about driver polarity is for naught.

Bottom line sounds like there is NO way to get these back to stock. Even with original drivers, the sound will be different due to aged materials, or due to reconing with inevitably different materials than stock.

My current thinking is:

• Prototype the speakers with crossovers mounted outside the box and perform measured and subjective testing of varying crossover configurations.
  
• Stuffing: Stick with 32oz fiberglass since that is what was used with old woofers. BUT, maybe stuffing has to be adjusted when crossover points change - anyone know?
  
• Inductor: Try both #5 and #7 but expecting #7 to be better (lower xover point) - if the AB midrange is closer to "new" 2AX response, then perhaps I will get more low end out of the mid, and can relieve the woofer a bit.
  
• Caps: Stick with 4uf and 6uf, but use new caps.
  
• Polarity: Keep mid and tweet with same polarity, but try reversing them w/ respect to woofer.
  
• Try some of the mods suggested for 3a, such as small inductor and resistor across the leads of tweeter/mid.
  
• Buy a copy of Xover-Pro, measure all drivers or replace with better units, and start from scratch? Will that do me any good or is it just an exercise in frustration?
  

Can anyone share a crossover design that will work with AB Tech 2AX replacement mid+tweet and original AR-2A woofers?

One other thought I am considering - what about remounting the tweeter and mid inline with the woofer? These are floorstanding units. Does this help with imaging, etc? Or is it blasphemy?